Induction generators are commonly used to generate electricity from wind. An induction generator includes a rotor, which provides a spatially varying magnetic field, and a stator, which has stator windings. As the wind turns the rotor, the magnetic field at the stator windings changes as a function of time. Since a time-varying magnetic field induces a time-varying voltage, this generates a time-varying stator voltage across the terminals of the stator windings.
A voltage waveform provided to a power grid must meet certain requirements. For example, the voltage waveform should have a frequency that strays by only a limited amount from a target line-frequency. In addition, the harmonic content of the voltage waveform should remain below a specified upper limit. Because wind speed varies considerably, the rotor will not necessarily rotate at a constant angular velocity. Thus, without some sort of correction, the voltage waveform provided by a wind-powered induction generator may vary considerably with wind speed.
To provide greater control of the waveform despite the varying winds, one can use active circuitry to control the waveform provided to the power grid. The active circuitry includes an AC/DC converter that converts the varying output of the stator into DC, and a DC to AC converter to take that DC waveform and convert it into the desired waveform.
One way to connect an induction generator to a power grid is to connect the stator directly to the power grid, and to connect the rotor to the power grid through active circuitry. A controller for controlling the active circuitry receives input indicative of both the angular frequency of the rotor and the waveform provided to the power grid. This enables the controller to provide feedback control over the current on the rotor. Based in part on these inputs, the controller adjusts the slip angle between the stator and the rotor. Since the stator windings respond to the rate of change in the magnetic field, and since the magnetic field is generated by current on the rotor, one can, by properly controlling that current, cause the stator windings to respond as if the rotor were turning at a constant angular velocity. An induction generator connected to the grid as described above is often referred to as a “doubly-fed induction generator.”
An advantage of the doubly-fed induction generator is that the bulk of the power provided by the generator bypasses the active circuitry. This both avoids incurring losses in the active circuitry for the bulk of the power, and avoids having to provide active circuitry that is rated to handle the entire output of the generator. In a typical installation, the active circuitry handles about 20% of the total output of the generator. The remaining 80% bypasses the active circuitry altogether.
Another method for connecting an induction generator to a power grid passes the stator voltage through active circuitry. An induction generator connected to the grid as described above is often referred to as a “full-converter induction generator.”
In a full-converter induction generator, the active circuitry is rated to handle the generator's entire power output. One disadvantage of the full-converter induction generator arises from the considerable losses sustained as a result of the conversions from AC to DC and back again. On the other hand, a full-converter induction generator generally provides better control over the characteristics of the stator voltage.
In the past, when wind turbines were not so common, the greater variations in the stator voltage associated with the double-fed induction generators would have an insignificant effect on the power grid. As wind turbines have become more popular, this is no longer so. As a result, many utilities have begun to require higher quality power output from wind turbines. This has made it necessary to replace double-fed induction generators with full-converter induction generators. The process of replacing double-fed induction generators with full-converter induction generators is time-consuming and expensive.